The present invention relates to expansion of tobacco by intense convection heating of moisture-containing tobacco, in a gas which may have steam as its major component, and to apparatus particularly well suited for that purpose.
U.S. patent application Ser. No. 304,713, entitled Process for Expanding Tobacco with Water, describes a process wherein moisture containing tobacco is very rapidly heated by means of a high speed downdraft of hot gas while said tobacco is transported on a conveyer with a porous belt. Said conveyer is therefore a feature of said process, and not just a means of transport.
U.S. Pat. No. 3,957,063 illustrates the use of two different types of conveyers in tobacco processing; vibrating and belt types. An updraft gas flow is directed through said vibrating conveyer, which has a porous pan. Said belt conveyer, however, is of conventional construction with its belt suspended between and driven by multiple rollers, and it is not porous.
U.S. Pat. No. 4,091,824 illustrates a belt conveyer, with a porous belt but also suspended between and driven by multiple rollers.
U.S. Pat. No. 3,982,550 describes a typical prior art dryer for the expansion of moisture-containing tobacco by means of very rapid heating. Said tobacco is heated by a hot gas while said tobacco is in pneumatic transport in a flow of said gas.
There are economic and smoking product design reasons for expanding tobacco in several different forms, e.g. stem and lamina. A process based on that described in U.S. patent application Ser. No. 304,713 is usable with virtually all of said forms of tobacco, and said process is operable with a wide variety of process conditions. Said process conditions may be extreme including high temperature and very short residence time, with said short residence time leading to a requirement for relatively high speed of said porous conveyer belt.
Said "belt" must further be of complex construction including a layer porous to gas flow but capable of supporting and restraining tobacco particles from moving with said gas. Said porous layer must either be structurally strong or must be supported on a second layer that does provide said structural strength. Moreover conventional lubrication is precluded by purity requirements of said tobacco, and some synthetic bearing materials, that could withstand both said high temperature and the absence of said lubrication, are likewise precluded, while said multiple layers will cause greater than usual internal friction.
There is therefore a need for a process conveyer, for use as a downdraft conveyer expansion dryer, that can operate reliably, unlubricated at high speed and temperature, providing very short residence time.